Nanostructuring improves the coupling of dielectric waveguides with plasmonic nanoresonators

Certain metallic nanostructures exhibiting localized surface plasmon resonances (LSPR) are capable of sensing extremely low-volume analytes down to attoliters, especially when used in a single particle configuration. Incorporating them into integrated photonics sensing platforms could result in a reduced limit of detection (LOD), and increased dynamic range and multiplexing capabilities. Despite the potential of this platform, several challenges remain, like low coupling efficiencies between integrated waveguides and plasmonic nanoantennae, and the need for off-chip readout. We numerically investigate the optical response of phase shifted Bragg grating (PSBG) and sub-wavelength grating (SWG) waveguides loaded with plasmonic nanoresonators in silicon nitride (Si<inf>3</inf>N<inf>4</inf>) integrated photonics platform operating in an aqueous environment. In comparison with a strip waveguide, a 3-4 times improvement in coupling, up to 5 times improvement in local intensity enhancement and 6-7 times improvement in intensity-shift sensitivity are predicted for the structured waveguide configurations. In particular, the PSBG configuration exhibited slightly improved coupling and intensity-shift sensitivity compared to the SWG configuration. On the other hand, the device footprint of the SWG configuration was only a fifth of that of PSBG and also exhibited nearly two times larger local intensity enhancement. A systematic study of the design space and sensitivity analysis is performed to assess the optimal configuration for single-ID single-wavelength refractometric sensing, on-chip excitation and off-chip readout, and SERS sensing.